The early method of signal transfer from static surface to rotating surface was through the use of carbon brushes, spring loaded contractors, gold slip rings or ball bearings. These methods however, lost ground with time-because of frictional wear, noise generation etc.
The modern method to achieve the above objective is with a rotating transformer normally used in video cassette recorders (VCR's). It consists of two wire wound discs of ferrites. One wire wound disc is connected on the stator (primary) and second were wound disc is connected on rotor (secondary) with windings facing each other. The distance between stator and rotor is very small. Signal transfer from static surface to rotating surface and vice-versa is via these primary secondary transformer based on magnetic flux linkage. It depends upon the current induced in the windings, ferrites used in the set up, turns ratio, air gap between primary and secondary etc.
Some Japanese companies are believed to be making such VCR rotating transformers. However, a major limitation is that these companies are only interested in supplying these items in bulk quantities. In addition VCR cores are not available separately and ferrites cannot be trimmed by machining. The companies do not provide their characteristics and other design details etc. The other sources of supply of these items are generally not known.